(a) Field of the Invention
The present invention concerns the preparation of an atmosphere under pressure for the manufacture of so-called high performance composite parts of the type wherein said atmosphere is used in an autoclave above a superposition of tissue cutouts with fibrous structure impregnated with a thermosetting organic material disposed between a lower mold and upper sealing covering member, the space between the covering member and the mold being preferably under vacuum.
(b) Description of Prior Art
In practice, these elementary fibrous structures in the form of pre-impregnated textile cutouts are made from fibers which are reinforced with graphite carbon, aramide, glass, while the thermosetting organic materials are epoxy, polyester, phenolic, vinylester, polyimide resins.
Molding is carried out in an autoclave under relatively high temperature, of the order of 150.degree. C. to 350.degree. C. and under a pressure of the order of 5 bars to 20 bars depending on the nature of the organic material used, the vacuum surrounding the structure during molding being a primary vacuum, enabling to eliminate with the help of the pressure which is maintained above the covering member, any residual gaseous bubble in said structure, and any residual presence of organic solvent.
The atmosphere which is used to produce a pressure and which is also useful for thermic transfer, currently comprises air, since under the conditions of pressure and temperature, there is no risk of spontaneous ignition of the organic material, nor of any propagation of the flame. In spite of this relative safety, some accidents which lead to an ignition have been observed and the analysis which has been carried out has established that there is always a cause which is not directly connected to the structure during the molding. It may be a short in the accessory electrical equipment, or a defect in the preliminary drying of the solvents for the organic material, and the ignition, at the start, can have some effects on manufacturing accessories such as the covering member, the tissue and the impregnation carpet, the sealing compound, etc.
As long as this so-called autoclave molding process with a covering member under vacuum was used with small parts, such accidents could be controlled without too much damage, both with respect to material used and people associated with the process.
However, this technology has recently been broadened because parts whose dimensions increase more and more are actually manufactured according to this process and it is not rare to use autoclaves with capacities of many hundreds of cubic meters. And this dimensional development is also accompanied by increased productivity requirements for such heavy equipments and autoclaves. On the other hand, certain resins which are not currently used, such as polyimides, for example the one known under the commercial designation PMR 15, require conditions of higher pressure and temperature, therefore resulting in increased hazards. Moreover, and obviously for good reasons, safety rules concerning employees provide for norms which are increasingly demanding.
This is the reason why it has been proposed to replace the air making up the atmosphere of the autoclave with inert nitrogen, but this represents a substantial increase of the investment (stocking of liquid nitrogen and evaporator, because of the large quantities of inert gas to be used in autoclaves of increasing sizes under increasingly higher pressures) and of the cost of utilization (since the liquid nitrogen obtained by cryogenic distillation is relatively expensive). Thus, to reach a compromise between cost and safety, a mixed solution was used wherein air is mixed with cryogenic nitrogen, while, however, remaining largely below the admissible theoretical value of the content of oxygen, but this solution has still proven to be expensive, and not very practical because of the requirements which are inherent in the technology of mixing gases.
The Applicant has undertaken a thorough study of the safety aspect of the composition of the atmospheres which can be used in this type of technology. Thus, the starting point was a maximum oxygen content (called oxygen index IO) which prevents the propagation of a flame after local ignition on a product in contact with said atmosphere, and if the real oxygen content is lower than IO, then the atmosphere qualifies as a security atmosphere for the product in question. The method used consisted in determining the oxygen index IO at a temperature TO of 25.degree. C. and a pressure PO of 1 bar and to proceed to an extrapolation by corrective calculation for higher pressures and temperatures (it should be noted that the correction of the pressure is in fact completely negligible). And, the experimental observation was to the effect that notwithstanding the type of polymer, the oxygen index IO, at 25.degree. C. under 1 bar, is always higher than 0.15. The result of this study is given herebelow:
______________________________________ T IO (1 bar) IO (30 bars) ______________________________________ 25.degree. C. 0.15 100.degree. C. 0.107 0.101 150.degree. C. 0.088 0.084 180.degree. C. 0.080 0.075 230.degree. C. 0.068 0.064 315.degree. C. 0.054 0.051 450.degree. C. 0.039 0.0377 ______________________________________
From a reading of these results, it will be observed that in the case of epoxy resins heated at at temperature of 180.degree. C., the oxygen index IO is 0.080 (1 bar), 0.075 (30 bars), while it decreases to 0.054 (1 bar), 0.051 (30 bars) at a temperature of 315.degree. C., which is the one used with polyimides.
These figures should be compared with the oxygen indices measured in an atmosphere in contact with various materials of accessory devices which can be used in a molding operation in an autoclave under a covering member under vacuum. As previously, this oxygen index corresponds to the minimum content of oxygen in a mixture of oxygen and nitrogen under atmospheric pressure at which propagation of the combustion of the material takes place naturally:
-molding operation at a temperature of the order of 120.degree. C. to 180.degree. C.
______________________________________ (polymerization of epoxy resins) Ref. TYGAVAC IO ______________________________________ Covering member NBF 205C 0.43 under vacuum 50.mu. Unmolding film RF 260 0.90 25.mu. Draining carpet NW 339HA 0.24 Unlaminating tissue 80 A/R 0.19 Unmolding film RF 239 0.27 ______________________________________
-molding operation at a temperature of the order of 300.degree. C.
______________________________________ (polymerization of polyimides/PMR 15) Ref. TYGAVAC IO ______________________________________ Covering member PBF 400 0.39 under vacuum 50.mu. Unmolding film RF 305 0.99 25.mu. Draining carpet NW 450HU 0.82 Unlaminating tissue 300 C/R 1 Sealing compound VRS 600 0.20 ______________________________________
The Applicant has then set up a margin of safety by proposing to limit the oxygen content to 80% of the oxygen index IO and has then concluded that an atmosphere in the presence of an epoxy resin at 180.degree. C. should have an oxygen content lower than 0.06 (6% oxygen), an atmosphere in the presence of phenolic resin at 230.degree. C. should have an oxygen content lower than 0.045 (4.5% of oxygen) and an atmosphere of polyimide resin at a temperature of 310.degree. C. should have an oxygen content lower than 0.040 (oxygen content lower than 4%).
Once these results were confirmed by experimentation, the Applicant tried to solve all these safety operating conditions as cheaply as possible. In this context, it should be noted that the problem to be solved included a priori the obligation to find a source of gas, in which the production cost was clearly lower than cryogenic distillation nitrogen, enabling in a simple manner without requiring expensive means to go from a safety atmosphere for a type of resin to another safety atmosphere for another type of resin, since many thermosetting resins with different ignition properties may be used in succession in the same treating installation. A solution, of course, would have been to select the safety atmosphere for the resin which is more highly inflammable (i.e. as was shown in the case of polyimide resin), but this would have led to totally inadmissible treatment costs for the less inflammable resins.